Spectral Response of the Pulsationally-Induced Shocks in The

Mon. Not. R. Astron. Soc. 000, 1–12 (2002) Printed 4 November 2018 (MN LATEX style file v2.2) Spectral Response of the Pulsationally-Induced Shocks in the Atmosphere of BW Vulpeculae Myron A. Smith1⋆ and C. Simon Jeffery2† 1 Computer Sciences Corporation/STScI, 3700 San Martin Drive, Baltimore, MD 21218 2 Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland Accepted . Received . ABSTRACT BW Vul is remarkable for exciting an extremely strong radial pulsation mode. This instability grows in its outer envelope and forms visible shock features in the continuum flux and spectral line profiles at two phases separated by 0.8 cycles. Material propelled upwards energetically in the atmosphere from the shock returns to the lower photosphere where it creates a second shock just before the start of the next cycle. We have obtained three nights of echelle data for this star over about five pulsation cycles (P = 0.201 days) in order to evaluate the effects of atmospheric shocks on a number of important red lines in the spectrum. These lines include He I λ5875 and λ6678, CII λλ6578-83 doublet, and other moderate (e.g., SiII λ6371) and high excitation (Si III λ5737) lines. We have added to these data 37 archival IUE/SWP echelle spectra obtained in 1994. We have investigated the equivalent widths and shapes of the optical lines for evidence of inter alia lags and have compared our results to the IUE fluxes extracted from the far-UV continuum, He II λ1640, and several resonance lines. A comparison of HeI λ5875 and λ6678 line profiles during the peak of the infall activity suggests that differences in the development of the blue wing at this time are due to heating and a short-lived formation of an optically thin layer above the region compressed by the infall. This discovery and the well-known decreases in equivalent widths of the C II doublet at the two shock phases leads us to suggest that shock heating flattens the atmospheric temperature gradient, whether it is the infall shock preferentially heating of the upper atmospheric layers from infall, or the pulsational arXiv:astro-ph/0210189v1 8 Oct 2002 wave shock, which takes on an isothermal character as it emerges into the more tenuous upper photosphere. Except for evidence of wind in the far blue wings of the UV resonance lines, we find no evidence for a shock delay arriving at different regions of line formation of the photosphere (i.e., a “Van Hoof effect’). Phase lags attributed by some former observers may be false indicators arising from varying degrees of desaturation of multiple lines, such as for the red He I lines. In addition, an apparent lag in the equivalent width curve of lines arising from less excited atomic levels could instead be caused by post- shock cooling, followed by a rebound shock, as suggested by subtle variations in the photospheric λ1640 and UV continuum flux curves. Key words: stars: variable: other – stars: individual: BW Vul – line profiles – line formation – shock waves. 1 INTRODUCTION tion mode (Stamford & Watson 1981, Aerts 1995) is so strongly excited as to produce discontinuous “standstill” The β Cephei variable BW Vul (HR 8007, HD 199140; B1 V features in the star’s light curve and, immediately following to B2 III) is in kinematic terms the largest amplitude pul- this, a longer standstill in the radial velocity curve as well. sator known in the Galaxy. Its fundamental radial pulsa- These features result from highly nonlinear processes associated with upward propagating pulsation waves. These waves emerge into the photosphere as highly supersonic shocks. ⋆ E-mail:[email protected] During the pulsation cycle, the optical line profiles remain † E-mail:[email protected] c 2002 RAS 2 M. A. Smith and C. S. Jeffery in absorption but undergo extreme variations in shape and (several percent of a stellar radius) as well as its virtual free velocity. Equivalent width variations are also noticeable at fall from maximum to minimum radius. certain phases. In the often-used convention that φ = 0 oc- In the most recent kinematical description, Mathias et curs at light maximum (minimum radius), the radial velocity al. (1998) and Garnier et al. (2002) have summarized the standstill becomes centered at φ = 0.98-1.00. Line profiles present consensus that there are two shocks per cycle. The exhibit double lobes at phases just before (centered at φ ≈ first, “pulsation,” shock is the result of the evolution of the 0.90), and during some cycles just after (φ ≈ 0.06) the veloc- upward-propagating wave which grows in amplitude from ity standstill phase (e.g., Mathias et al. 1998). Adding to the the envelope where it is excited. As it emerges into the at- complexity of description, the radial velocity curve is some- mosphere at φ = 0.1, this shock has a moderately high Mach what sensitive to the method of measurement, the spectral (5-7) number, as referenced by the velocity “discontinuity” and temporal resolution of the observation, and especially just prior to the velocity standstill. A subsequent, “infall,” to the momentary pulsational amplitude of the star, for the shock, occurring 0.8 cycles after the first, is due to the ex- amplitude of the pulsations fluctuate by several percent from treme compression of the upper atmospheric strata as they night to night (Crowe & Gillet 1989, Aerts et al. 1995, Math- fall back and catch up to the slower moving layers of the ias et al. 1998, Garnier et al. 2002). The equivalent widths of lower photosphere. In this picture the line profiles exhibit some metal lines vary with phases as a function of excitation double lobes during the main (and often infall) shock be- potential (Furenlid et al. 1987). The finite signal-to-noise cause of the velocity jump associated with it. Mathias et ratio and temporal sampling frequency of the International al. also note that because the density of the atmosphere de- Ultraviolet Explorer (IUE) observations set practical limits creases monotonically outwards, the infalling region cannot on the otherwise considerable complementary UV informa- be described as a disconnected shell. In addition, Mathias et tion that they offer to optical spectra. al. suggested that shock progresses inward in terms of ab- Despite these observational limitations, important ef- solute (Eulerian; radius from star center) coordinates even fects of the star’s pulsation cycle are readily visible on the as it moves outwards in mass. Thus, their description recon- atmosphere. One of these is a variation of the effective sur- ciles the idea expressed by several previous authors that two face gravity and especially the instantaneous “effective tem- outward-moving shocks per cycle propagate up through the perature” of the star during the cycle. Recently, Stickland atmosphere. In the past the infall shock, which forms at φ ≈ & Lloyd (2002) have compared flux variations at a range of 0.90, has been mistaken for a reflection of a shock from the wavelengths from the far-UV to the near-UV to show that previous cycle off an interior density gradient discontinuity. the effective temperature varies from 20,000 K to 25,000 K In this study we adopt the view of Mathias et al. that this during the cycle. Temperature variations this large may well shock is a natural consequence of infall, and that any ear- cause observable modulations in the mass loss and X-ray lu- lier reflected shock is likely to be damped within the star, minosity (cf. Cohen 2000) of the cycle. rendering it invisible at the surface. Historically, controversy has surrounded the interpreta- The elusiveness of even a qualitative interpretation of tions of the profile and strength variations caused by shock the shocks in BW Vul has slowed the necessary develop- waves moving through the atmosphere of BW Vul. One of ment of self-consistent radiative hydrodynamical models. these is the so-called “Van Hoof effect,” named after its pri- Early on, Stamford & Watson (1978) assumed that a large- mary discoverer (Van Hoof & Struve 1953). This effect is the amplitude velocity piston at the base of the atmosphere purported phase lag between the velocity curves extracted developed into a thin, isothermal shock as it progressed from lines formed at different atmospheric depths. This is through the line formation region. Using this dynamical thought to be the result of the finite travel time required for model atmosphere, they constructed line profiles of Si III a pulsational shock wave to move up from one region of line λ4552 at several key phases in the cycle. Profiles at phases formation to another. In the most recent such report, Math- we now call the infall shock exhibited line doubling (albeit ias et al. (1998) reported that double line profiles of various over only a brief interval). In subsequent work Stamford & lines observed near φ ≈ 0.9 and sometimes 0.1 can exhibit Watson (1981) placed a large, adiabatic sinusoidal velocity equal blue-red strengths at slightly different phases. A re- variation at the base of a gray model atmosphere and demon- lated issue is the cause of the line doubling itself. Odgers strated that an isothermal shock developed in the line forma- (1955) and Goldberg, Walker, & Odgers (1976) first at- tion region. Although they did not compute line profiles in tributed the velocity discontinuities to atmospheric absorp- these simulations, Stamford & Watson stated that they an- tions just below and above the shock. These authors argued ticipated that the shock would produce line doubling during that as an upward-propagating pulsation wave breaks into the shock passage. The 1978 Stamford & Watson paper to a shock it accelerates the line forming regions of the atmo- date represents the only attempt to compute the line transfer sphere from the lower photosphere, thereby creating a den- for a spectral line in a moving model atmosphere appropriate sity discontinuity with respect to the lower photosphere.

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